The invention concerns a thermo-transfer color ribbon for luminescent lettering or coding.
Modem sorting machines, as they are being employed for a multitude of objects, such as for instance, letters, respond to luminescent coding, which is not necessarily visible to the human eye. For that purpose, the to be sorted pieces are provided prior to the sorting process with symbols which contain luminescent material. Thermo-transfer color ribbons are increasingly being used for this purpose, which have a layer of thermo-transfer dye with luminescent pigment contained therein. The luminescent transfer dye, which is transferred to the substrate surface, is very thin and transparent for visual inspection.
Luminescent dyes have the property of absorbing ultraviolet light and visible light in the blue part of the spectrum and radiate the absorbed part at the lower end of the spectrum. From among the great number of organic compounds which radiate visible light under the effect of shortwave rays, only such substrates are suitable as luminescent dyes or lumogens which distinguish themselves in solid, non-dissolved state through intensive fluorescence. Of greatest technical interest are those luminescent dyes, which fluoresce colored in daylight and which are utilized as day-glow fluorescent pigments. Soluble dyes of this type are for example, Rhodamin, Eosin, brilliant sulfoflaven FF as well as the intensively yellow-green fluorescing pyranin, also color pigments, for example 2.2-dihydroxy-alpha-napthaldiazine and anthrapyrimidine. Since the dyes are organic in nature, it is necessary to dissolve them with an organic medium or carrier. One uses predominantly tinted carrier materials, for example, pulverized polymerisates, which have been tinted with soluble dyes or finely dispersed pigments. The material types which correspond to the requirements as a carrier or a matrix for the dyes are transparent organic resins. By reacting acid polyester resins with alkaline dyes or by pulverizing solidified dye solutions one likewise obtains tinted carrier substances. Urea formaldehyde resins, acrylic resins and melamine resins are also used as carriers, on which the dyes are lacquered on, where necessary. Day-glow fluorescent pigments are organic synthetic material particles, which are tinted with fluorescent dyes. The physical structure of the pigment particles is primarily amorphous. The day-light fluorescent pigments are sold on the market under the names Lumogens (BASF), Day Glo.RTM. Colors, Goldfire Colors, Fluorzink or Brillink-Glow-Colors.
Thermo-transfer ribbons have been known for some time. They have foil-like carriers, for example made of paper, of a synthetic material or similar, a thermo-transfer color, specifically in form of a synthetic-and/or wax-bonded dye-or a carbon black layer. In thermo-print technology, the thermo-transfer color is softened by means of a thermal print head and transferred to a substrate. Thermal printers or thermal print heads, which are utilized for this purpose are known, for example, from DE-AS 20 62 495 and 24 06 613 and also from DE-OS 32 24 445. The various steps of the process are detailed as follows: A letter is formed on the thermal print head of the printer, which consists of heated dots and is to be printed onto a piece of paper. The thermal print head presses the thermo-transfer ribbon on the printing paper. The heated letter of the thermal print head, having a temperature of up to approximately 400.degree. C., causes the thermo-transfer color to be softened at the heated spot and be transmitted on the piece of paper in contact with the same. The used portion of the thermo-transfer ribbon is then passed to a spool.
For printing, socalled serial printers or line printers can be used. The serial printers operate with a relatively small, movable print head of up to approximately 1 cm.sup.2. On it are arranged, in vertical direction to the writing direction, 1 or 2 rows of dots (dot=selective approach heating point). The dot diameter ranges from approximately 0.05 to 0.25 mm. The number of dots per line is between 6 and 64, which corresponds to a resolution of 2 to 16 dots/mm. It is typical with respect to the serial thermo head, that it is moved during the printing process in horizontal direction vis-a-vis the transport direction of the paper. In contrast to the serial print head, with respect to a line print head we are dealing with a stationary head or strip. Inasmuch as the print strip is not mobile, it must span across the width of the substrate which is to be printed. Resolution and dot size correspond to those of serial heads. When luminescent material is deposited on white paper, the whiteness of the paper serves as light reflector. The largest portion of the incident light is reflected back by paper through the printed luminescent material. The reflected light noted by the observer contains both incident light and also luminescence light.
If the luminescent material is transferred to the surface of a darkly colored paper, then a portion of the incident light, which has passed through the luminescent light, is absorbed by the paper. The amount of available light due to re-reflection is decreased. In addition, that portion of light emitted by the luminescence layer is being absorbed, which is radiated in the direction of the paper surface.
In order to compensate for the luminescence intensity differences, which is based on the type of carrier, DE-OS 30 42 526 proposes a fluorescent print ribbon, which is characterized by addition of a blocking material to the fluorescence pigment material, in order to block absorption of incident light in the medium, onto which the pigment and the blocking coating is transferred during printing. The blocking material is preferably transferred as second layer over the pigment material layer. Both layers are transferred to the substrate in reverse order during the printing process. The blocking material contains reflecting metal particles or mother-of-pearl type pigments.
DE-AS 12 22 725 discloses a transfer material for luminescent lettering with a coating support of paper or foil and a luminescent color layer arranged thereon, whereby a pigmented cover layer is positioned over the luminescent, light radiation reflecting color layer which participates in the writing process. The cover layer preferably contains titanium white and/or aluminum print etching powder.
The known suggestions are aimed at preventing an absorption in the substrate of the incident light, which passes through the luminescence layer, so that this part is reflected and again passes through the luminescence color layer, in order to thus increase total excitation yield. The disadvantage hereby is that the luminescence light noted by the observer is always mixed with the reflected part of the incident light. The luminescent print outs therefore always appear pale, i.e., they have a low optical density.
If one attempts to increase the optical density of the print-outs by addition of a non-luminescent pigment to the layer of the luminescent pigment, one notes that with an addition of extraneous pigments of more than 1%, fluorescence quality will be significantly affected. With increased additional amounts, the brilliance of the fluorescence pigments, fluorescence intensity and purity of color will increasingly be affected because of the appearance of interferences. Still larger additional amounts lead to an almost complete extinction of the fluorescence. From a fluorescence yield aspect, an acceptable additional amount of 1% or below would only insignificantly increase the optical density.